In conventional relay networks, messages are transmitted from a source node to a destination node via a single path with, perhaps, multiple serial hops through relay nodes. In a cooperative relay networks, wireless nodes cooperate with each other in transmitting messages in parallel. By exploiting the broadcast nature of a wireless channel to reach multiple relay nodes concurrently, and by enabling the relay nodes to cooperate, it is possible to reduce power consumption in delivering a message from the source to the destination. This can also significantly increase gains in overall throughput and power efficiency, A. Nosratinia, T. Hunter, and A. Hedayat, “Cooperative communication in wireless networks,” IEEE Communications Magazine, vol. 42, pp. 68-73, 2004, Sendonaris, E. Erkip, and B. Aazhang, “User cooperation diversity—Part I: System description,” IEEE Transactions on Communications, vol. 51, pp. 1927-1938, 2003, A. Jardine, S. McLaughlin, and J. Thompson, “Comparison of space-time cooperative diversity relaying techniques,” in Proc. IEEE VTC 2005—Spring, pp. 2374-2378, 2005, and J. N. Laneman, D. N. C. Tse, A. Stefanov and E. Erkip, “Cooperative coding for wireless networks,” IEEE Trans. Commun., pp. 1470-1476, September 2004, and G. W. Wornell, “Cooperative diversity in wireless networks: Efficient protocols and outage behavior,” IEEE Transactions on Information Theory, vol. 50, pp. 3062-3080, 2004, all incorporated herein by reference.
In cooperative networks, messages are transmitted to the destination node in parallel using several intermediate relay nodes, A. Wittneben, I. Hammerstroem, and M. Kuhn, “Joint cooperative diversity and scheduling in low mobility wireless networks,” IEEE Global Telecommunications Conference (GLOBECOM), vol. 2, pp. 780-784, 2004, A. E. Khandani, J. Abounadi, E. Modiano, and L. Zheng, “Cooperative routing in wireless networks,” Allerton Conference on Communications, Control and Computing, 2003, and Rankov and A. Wittneben, “Distributed spatial multiplexing in a wireless network,” The Asilomar Conference on Signals, Systems, and Computers, pp. 1932-1937, 2004, all incorporated herein by reference.
Some simple relay selection criteria are described by J. Luo, R. S. Blum, L. J. Cimini, L. J. Greenstein, and A. M. Haimovich, “Link-Failure Probabilities for Practical Cooperative Relay Networks,” IEEE Globecom 2005, incorporated herein by reference. Two of the criteria, ‘Pre-Select One Relay’ and ‘Best-Select Relay’ select a single best relay based on a mean channel gains, while in the remaining two criteria, ‘Simple Relay’ and ‘ST-Coded Relay’, all the relay stations that decode data from source are selected. In ‘Simple Relay’, the relay nodes do not synchronize their phase, while in ST-Coded Relay, a distributed space-time code is used.
Search algorithms for selecting a single relay node based on an average distance or path loss between the nodes, based on the frame error probability and pairwise code word error probability are described by Zinan Lin and Elza Erkip “Relay Search Algorithms for Coded Cooperative Systems,” IEEE Globecom 20005, incorporated herein by reference.
Khandani et al. describe a model that is restricted to additive white Gaussian noise (AWGN) channels with phase compensation. That model does not consider dynamic fading-induced channel variations, outage, or the overhead required for cooperation between relay nodes.
Knowledge of the channel state information (CSI) at a transmitter is assumed by Laneman, Rankov, and Larson above. However, they do not consider the cost of acquiring the CSI. Wittneben only considers amplify-and-forward, which also neglects the cost of acquiring the CSI, see also Abdallah and H. C. Papadopoulos, “Beamforming algorithms for decode-and-forward relaying in wireless networks,” Conference on Information Sciences and Systems, 2005.
If the relay nodes do not have the CSI, then the receiver can, at best, accumulate the mutual information from the various relay nodes, e.g., through space-time coding, see Luo et al. and Jardine et. al. Outage analysis of such relay schemes, when the links operate at a given signal-to-noise ratio, are described by Y. Zhao. R. Adve, and T. J. Lim, “Outage probability at arbitrary SNR with cooperative diversity,” IEEE Communications Letters, pp. 700-702, 2005, and A. Khisti, U. Erez, and G. Wornell, “Fundamental limits and scaling behavior of cooperative multicasting in wireless networks,” IEEE Trans. Information Theory, Vol. 52, No. 6, June 2006, incorporated herein by reference.
Cooperative communications in wireless networks can be achieved using a number of well known physical layer communication techniques. When the channel fading is unknown at the transmitter, space-time coding can improve the reliability of the links. Distributed space-time coding is another way of achieving diversity in cooperative communications over fading channels. When the channel coefficients are known to the transmitters, distributed beamforming, or distributed space division multiple access techniques can be used to achieve cooperative gains. In channels involving relay stations, decode-and-forward, amplify-and-forward, and bursty-amplify-and-forward schemes are the prevalent communication techniques and protocols.
Of particular interest is cooperative communication for the downlink of a cellular network, where during each scheduling interval, multiple messages are transmitted from a base station to mobile stations, e.g., cellular telephones. Scheduling in wireless networks is described generally by Andrews, “A survey of scheduling theory in wireless data networks,” in Proc. of the 2005 IMA summer workshop on wireless communications, June 2005, and Liu et al., “A framework for opportunistic scheduling in wireless networks,” Compute Networks, vol., 41, no. 4, pp. 451-474, 2003, all incorporated herein by reference.
There are two main categories of relay stations in the cellular setting: fixed relay stations and mobile relay stations. The benefits of employing fixed relay stations include extending cell coverage, boosting transmission rates, improving spectral efficiency, and costs relative to constructing complete base stations, Hu et al., “Range extension without capacity penalty in cellular networks with digital fixed relay stations,” in IEEE Global Telecommunications Conference (Globecom), November 2004, and Pabst et al., “Relay-based deployment concepts for wireless and mobile broadband radio,” IEEE Communication Mag., vol, 42, no. 9, pp. 80-89, September 2005, all incorporated herein by reference. Mobile relay stations are more likely to occur in ad hoc networks, all incorporated herein by reference.
Another cooperative strategy uses a MIMO fixed relay with linear processing to support multi-user transmission in cellular networks. Chae et al., “MIMO relaying with linear processing for multi-user transmission in fixed relay networks,” IEEE Trans. Signal Processing, vol. 9, no. 1, pp. 19-31, 2006, incorporated herein by reference. The single fixed relay processes the received signal with linear operations and forwards the processed signal to multiple mobile stations. They describe a two-phase protocol. Phase 1 uses a MIMO channel between a multi-antenna base station and a MIMO relay station, while during phase 2, the MIMO relay station pre-codes and transmits the messages to multiple mobile stations.
Challa et. al., “Cost-aware downlink scheduling of shared channels for cellular networks with relay stations,” in IEEE IPCCC, April, 2004, incorporated herein by reference, describe two hop downlink scheduling in cellular networks with relay stations. There, the scheduling algorithm tries to improve the shared channel utilization by selecting either a one hop path from base station to mobile, or a two hop path from the base station to the relay and then from the relay to mobile that yields the best channel throughput while using low transmission power for the shared channel in a CDMA based cellular system. However, cooperation between relay stations is not permitted.
Viswanathan et al., “Performance of cellular networks with relay stations and centralized scheduling,” IEEE Trans. Wireless Commun., vol. 4, no. 5, pp. 2318-2323, September 2005, incorporated herein by reference, describe a centralized downlink scheduling scheme in a cellular network with a small number of relay stations. They obtain throughput results for various scenarios and study the effect of number of relay stations, relay transmit power relative to the base station power, and the effect of distributing a given total power between the base station and different numbers of relay stations. There is no cooperation between relay nodes.
Cooperative transmission has also been considered from an information theoretic perspective under the name of cognitive radio channels, or interference channels with degraded message sets, see Devroye et al., “Achievable rates in cognitive radio channels,” IEEE Trans, Inf. Theory, vol. 52, no. 5, pp. 1833-1827, May 2006, and Jovicic et al., “Cognitive radio: An information-theoretic perspective,” IEEE Trans. Inf. Theory, May 2006, both incorporated herein by reference.